1. Field of the Invention
The present invention relates to a bright point detecting method and a computer program product for conducting the bright point detecting method in which a cross filter effect, one of spectacular effect filter, is applied to digital images shot by a digital camera or read by a scanner scanning a photographic film.
2. Description of the Related Art
A cross filter is one type of various spectacular effect filters used with a camera lens unit and implemented by a transparent optic filter with several slits provided in a surface. With the cross filter mounted to a silver halide photographic camera, a resultant photo has a luminous effect where a radial form of bright lines emit from the highlight on a subject to be shot.
The same effect may be obtained with a digital camera using such an optical cross filter. However, the imaging device in the digital camera is smaller in the light receiving area than the film size of a silver halide camera. Accordingly, as the digital camera employs a wide-view lens which is short in the focusing distance and long in the depth of view, it may record the slits in the cross filter when used.
In view of the above aspect, an image processing method is introduced for extracting highlight locations from the image produced by a digital camera and plotting the bright lines extending from each of the highlight locations. The imaging device or CCD in the digital camera is as low as 7 EV in the dynamic range to exposure light as compared with the conventional silver halide cameras. As a common image contains a difference in the luminance greater than the dynamic range, it may develop luminance saturated regions. It is hence difficult to extract the highlight on the subject at a considerable degree of accuracy.
For compensation, a modified digital camera is proposed as disclosed in Japanese Unexamined Patent Publication No. (Heisei)6-30373 which includes a read-in controlling means for, when the subject to be shot has been recorded at an optimum exposure level in a recording medium, reading the subject with its exposure level controlled to be declined by a predetermined step from the optimum exposure level and a record controlling means for detecting each location on the subject read by the read-in controlling means where the luminance is higher than a reference level and recording data of the location on the subject in a given area of the recording medium.
It is still disadvantageous that the optical cross filter on the silver halide camera has to pass a series of trial shooting actions for proving the spectacular effect desired and also its actions require a sort of skill as are not easily conducted by an ordinary personnel. Moreover, the process of eliminating from the image unwanted bright lines which have been recorded at the shooting is a highly intricate task.
It is more practical and functional for producing the cross filter effect to employ an image processing method of detecting a cluster of pixels or bright points in an image which are higher in the luminance than a threshold level and plotting the bright line along the contour of the pixel cluster. It is however difficult for the known image processing method to explicitly discriminate a light emitting object from other whiter objects in the image and detect it as the bright point. If worse, the bright line will appear on an unwanted object in the image. Also, the bright line is substantially linear and may be a chain of white or luminous color. Accordingly, when enlarged in size, the bright line appears in a zigzag form or not properly chained form. It is thus desired to modify the image processing method for producing an improvement of the cross filter effect.
In case that the known image processing method is applied to any type of digital camera which employs none of the mentioned-above function disclosed in the Publication, its unwanted effect will be emphasized because the actual dynamic range of the image has hardly been recorded. It is thus desired to provide an improved image processing method for detecting the bright point at higher accuracy and plotting a more natural form of the bright line.